**yard (yd) [1]**- a traditional unit of distance equal to 3 feet
or 36 inches. The word comes from a Saxon word
*gyrd*or*gyard*meaning a stick, although the unit known as the*gyrd*in Saxon times was actually the rod [1], not the yard. The yard was established after the Norman conquest of 1066. According to tradition, King Henry I decreed that the yard should be the distance from the tip of his nose to the tip of his outstretched finger, thus defining the yard as exactly 1/2 fathom. Whether this actually happened or not, it does seem that the yard and the English foot were set at close to their modern lengths during or around the time of Henry's reign (1100-1135). The length of the oldest known standard yardstick, believed to date from 1445, agrees with the modern length within less than 0.1 millimeter. Today one yard is officially equal to exactly 91.44 centimeters or 0.9144 meter; this definition was adopted in the U.S. in 1959 and in Britain by the Weights and Measures Act of 1963. **yard (yd) [2]**- the yard is traditionally used as a unit of area for materials sold in standard rolls, such as cloth, carpet, linoleum, fencing, and so on. In each case, one yard represents an area one yard long and as wide as the roll width. See bolt.
**yard (yd) [3]**- a cubic yard (about 764.6 liters). Bulk commodities like sand or topsoil are often sold by the "yard."
**yard of ale**- a traditional Scottish measure of volume. A yard of ale is roughly 2.5 pints (1.4 liters) served in a slender glass one yard [1] tall.
**yard of land, yardland**- one of several traditional units of area in old England; a "yard of land" sometimes meant a virgate (roughly 30 acres) and sometimes a rood (1/4 acre).
**year (a or y or yr) [1]**- a unit of time, defined to be the period of time required for the Earth
to make one revolution around the Sun. To be more precise, the year we use
in ordinary life (described in the next entry) is designed to approximate
the interval between two arrivals of the Sun at the Tropic of Capricorn,
marking the summer solstice in the Southern Hemisphere and the winter solstice
in the Northern Hemisphere. Astronomers call this unit the
**tropical year**. There are 365.242 199 days in a tropical year, or, to be even more precise, 31 556 925.9747 seconds. Since the symbol*yr*is specific to English, the symbol for the year often used in scientific writing or other international contexts is*a*, taken from the Latin word,*annus*. Thus 1 Ma stands for a million years and 1 Ga for a billion years. **year (a or y or yr) [2]**- a traditional unit of time usually equal to 365 or 366 days. We need a whole
number of days for the
*calendar year*used in ordinary life. Ancient astronomers knew that the year [1] is approximately 365 days long, and we now know the correct figure is approximately 365.242 days. If we use 365 as the number of days in every calendar year, the extra 0.242 day adds up quickly and causes large errors in predicting the seasons. To solve this problem, the Roman emperor Julius Caesar decreed in 46 BC that the calendar year should have 365 days generally, but that every fourth year should have an extra, or 366th, day. The longer year is called a**leap year**. In this**Julian calendar**, four years equal exactly 1461 days, so the average**Julian year**is exactly 365.25 days.

This was a big step toward accuracy in the calendar, but the Julian year is too long by 0.008 day, or a little over 11 minutes. By the time of the Renaissance, these 11-minute errors had accumulated to a total error of about 10 days (since the Council of Nicaea in 325 AD, which set the rules for deciding when Easter should be celebrated). The spring equinox was occurring near March 11 instead of March 21. In 1582, Pope Gregory XIII decreed that 10 days should be dropped from the calendar: the day after 1582 October 4 was October 15. To reduce future errors, the pope further decreed that years divisible by 100 are not leap years unless they are also divisible by 400. Thus 2000 and 2400 are leap years, but 2100, 2200, and 2300 are not. It took many years, but the Gregorian calendar has now been accepted as the civil calendar in all countries of the world.

With the Gregorian adjustment, there are exactly 146 097 days in every 400 years, and the average**Gregorian year**is exactly 365.2425 days. The Gregorian year is still too long, but by less than half a minute. It will take thousands of years for this error to accumulate to 1 day, so the calendar year and the tropical year are in good enough agreement to last us a long time.**yearhundred**- an old and rarely used name for the century (100 years). This unit is the English counterpart of the word for a century in other Northern European languages such as German (
*Jahrhundert*) and Swedish (*århundrade*). **yi**- a unit of quantity in Chinese, equal to 100 000 000. The yi is used with the wan (10 000) in expressing large numbers, one yi being the same as one wan wan (ten thousand ten-thousands).
**yo or y/o**- a common symbol for "years old."
**yocto- (y-)**- a metric prefix denoting 10
^{-24}(one septillionth). Adopted by the CGPM in 1990, the prefix is derived from the Latin*octo*and Greek*okto*, meaning 8, because this is the eighth prefix (n = 8 in 10^{-3n}) in the SI system of metric prefixes. The*y*was added arbitrarily to provide a non-confusing letter for abbreviations. **yoke**- another name for a pair. The yoke is used in describing teams of animals, especially oxen, used to pull plows or wagons.
**yotta- (Y-)**- a metric prefix denoting 10
^{24}(one septillion). The prefix was coined to parallel the prefix yocto-. **yottameter (Ym)**- a metric unit of distance equal to 10
^{24}meters or 10^{21}kilometers. One yottameter equals 32.408 megaparsecs (Mpc) or 105.7 million light years. We ought not to need longer distance units than this, because the radius of the observable universe is not more than about 200 yottameters.

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May 14, 1999